1. Field of the Invention
The present invention relates to ferroelectric liquid crystal shutters for use in three-dimensional stereoscopic televisions, frequency-modulatable stroboscopes, etc.
2. Description of the Prior Art
Liquid crystal shutters employing nematic liquid crystals usually have a response speed of several tens of milliseconds. Such shutters are known which have improved fall time due to a twist through 270.degree., or are adapted to have a response speed of about 1 msec by the two-frequency drive method. However, to obtain a shorter response time, it is suitable to use ferroelectric liquid crystals proposed by Clark and Lagerwall in (Applied Physics Letters, Vol. 36, p. 899, published in 1980. Ferroelectric liquid crystals generally in use are smectic C* phase liquid crystals which already afford response speeds of several tens of microseconds at room temperature.
The ferroelectric smectic C liquid crystal proposed by Clark et al. has a helical structure. When a cell having the liquid crystal enclosed therein is greatly decreased in thickness, the helical structure disappears, leaving only two domains which differ in optical axis. The ferroelectric liquid crystal molecules have spontaneous polarization, can be switched over between these two domains by the application of d.c. pulses and have such a memory effect that even when the voltage is turned off after the switching pulse application, the resulting state can be retained. Attempts are made to utilize these characteristics in liquid crystals of this type for use in liquid crystal shutters and large-capacity displays.
However, with liquid crystal shutters employing ferroelectric liquid crystals, the cell is likely to have two optical axes because the liquid crystal molecules therein include those having an UP state and a DOWN state due to spontaneous polarization. Generally, the optical axes can be lined up in either the UP state or DOWN state by the application of an electric field, whereas if there is zigzag disclination which is an orientation defect, there appears on opposite sides of the disclination a domain which is more stable in the UP state and a domain which is more stable in the DOWN state, and the optical axes of the cell are out of alignment.
Generally with liquid crystal shutters which are subjected only to on-off repetitions as specified, a rectangular wave is applied for driving. In the case of this waveform, however, unnecessary high d.c. voltage is continuously applied even after switching, seriously impairing the reliability of the cell. The result appears as a disturbance in orientation. Further, when a memory waveform is used for driving, in the cell which is prepared by rubbing, generally the liquid crystal becomes twisted, or the optical axis between the liquid crystal molecules being in the memory state fails to shift through an angle as large as the tilt angle of the liquid crystal, failing to give a high contrast.
Accordingly, there exists a driving method which is adapted to greatly shift the optical axis to give a high contrast by applying d.c. bias in order to maintain the optical axes after the application of a switching pulse. When the upper and lower orientation films having the same quality and treated by rubbing are provided on the respective two glass substrates for forming the cell, the cell optical axes in the UP and DOWN states are positioned generally symmetrically with respect to the rubbing axis, whereas the above method shows the drawback that when the drive voltage is applied for switching, variations in the d.c. bias for maintaining the axes shift the two axes of the cell. The shift of the two axes presents extreme difficulties in positioning the polarizing plate, further making it impossible to afford a high contrast with good stability.